Induction of embryogenesis using cytoskeleton inhibitors or protein synthesis inhibitors

ABSTRACT

Embryogenesis from plant microspores is routinely induced with a 16-24 h temperature treatment of 32.5° C. Continuous culture at 25° C. results in pollen development. However, microspore treatment with anti-cytoskeletal agents, or protein synthesis inhibitors, at the non-inductive temperature of 25° C., can induce embryogenesis, thus demonstrating that heat shock is not required for embryogenic induction. Furthermore, when anti-microtubule agents (e.g. colchicine) are used, embryo induction and chromosome doubling occur simultaneously, thus generating doubled haploids, whereas heat induction generates haploids. Thus, the use of microtubule inhibitors will provide a simple one-step process to simultaneously induce embryogenesis and chromosome doubling for the production of fertile plants, thus providing minimal manipulation which will be very advantageous for genetic studies and plant breeding programs. As noted, heat shock induces haploids. A low level of chromosome doubling can be obtained by adding colchicine to microspore cultures during the heat treatment. However, the use of trifluralin with the heat treatment, to generate doubled haploid plants results in an improved recovery of fertile doubled haploid plants than previously shown in the prior art.

FIELD OF THE INVENTION

The present invention is directed to the use of cytoskeleton modifiersor protein synthesis inhibitors to induce embryogenesis from plantmicrospores.

BACKGROUND AND PRIOR ART

The production of haploid plants generated through either anther orisolated microspore culture has succeeded in over 240 species from 85genera in 38 families (Srivastava and Johri 1988). Microspore culture ofBrassica napus has become one of the most efficient embryogenic systemsand has been exploited for developmental studies (e.g. Zaki andDickinson 1991; Telmer et al. 1992, 1993, 1994), for mutagenesis andgene transfer (Swanson et al. 1989; Huang 1992), and for development ofdoubled haploid homozygous breeding lines (Chen and Beversdorf 1992).The use of haploid plants, generated from anther or microspore culture,has enhanced the efficiency of crop improvement programs (Collins andGenovesi 1982, Chen and Beversdorf 1992). Although haploid plants can bereadily regenerated, the haploids cannot be used directly in geneticstudies and breeding programs because they are sterile (Subrahmanyam andKasha 1975). The current methods of doubling the chromosome complementof haploids to produce fertile homozygous doubled haploids areinefficient and labor intensive.

Efficient induction of embryogenesis is necessary for developmental andbiochemical studies. The efficiency of embryogenesis of B. napus hasbeen improved by using donor plants grown at low temperatures (Keller etal. 1986), by optimizing the microspore culturing conditions (Keller etal. 1986; Lichter 1981; Fan et al. 1988; Chuong and Beaversdorf 1985;Kott et al. 1988; Gland et al. 1988; Huang et al. 1990) and by usingmicrospores at the competent developmental stages (Telmer et al. 1992).

Exposure of microspores to a high temperature (32.5° C.) is consideredto be a key factor for induction of embryogenesis (Keller and Armstrong1978; Cordewener et al. 1994) and it has been proposed that heat shockproteins play a role in the inductive process (Pechan et al. 1992).Several unique proteins, synthesized during heat induction, have beenidentified and it has been suggested that they may be early markers ofembryogenesis or heat shock proteins involved in the induction process(Cordewener et al. 1994). However, with the use of heat shock to induceembryogenesis, it is very difficult to distinguish between factorsassociated with the heat shock process and those specific to theembryogenic process. Attempts to replace heat induction with alternatemethods such as gamma irradiation or ethanol treatments resulted in verylow embryo induction (Pechan and Keller 1989). However, the induction ofsporophytic development, by means other than heat, would be veryvaluable to allow discrimination of heat shock factors and embryogenicfactors and thereby identify the critical events involved in the changefrom gametophytic to sporophytic development.

Microspore morphology is altered by the 32.5° C. heat treatment.Although several morphological changes have been identified in B. napuscv. Topas, including the appearance of cytoplasmic granules andorganelle-free regions, plasma membrane associated electron-densedeposits, and microtubule reorganization, the most prominent change isthe dislocation of the nucleus (Fan et al. 1988; Simmonds et al. 1991;Telmer et al. 1993, 1994; Simmonds 1994). During pollen ontogenesis thenucleus of an early and mid-unicellular (MU) microspore is centrallylocated; during vacuolar enlargement, it is relocated to a lateralposition, the unicellular-vacuolate stage (UV); and it remains appressedto the edge of the cell in the late-unicellular (LU) stage after thedisappearance of the large vacuole (Telmer et al. 1992, 1993). The LUmicrospore enters the first pollen mitosis which is acentric and resultsin an asymmetrical division comprising a small generative cell and alarge vegetative cell separated by an unstable cell wall (Telmer et al.1993). If the LU microspore is subjected to the heat treatment, thenucleus migrates to a more central position where mitosis occurs andultimately results in a symmetrical division with two daughter cellssimilar in size and organelle distribution, and separated by a stablecell wall (Fan et al. 1988; Telmer et al 1993; Simmonds 1994); thesymmetric division blocks further pollen development and identifies theinduced structures (Telmer et al. 1994). An early structural markerwhich predicts a change in microspore division symmetry is a preprophaseband (PPB) of microtubules; the PPB, a cortical ring of microtubules,appears in the medial region of the microspore after only about 6-8 h ofheat treatment (Simmonds et al. 1991; Simmonds 1994). PPBs have not beenobserved during pollen development (Van Lammeren et al. 1985; Terasakaand Niitsu 1990; Simmonds et al. 1991). As the PPBs predict the positionof the future division plane in organized (Gunning and Hardham 1982) anddisorganized tissue (Simmonds 1986), and may have a role in wallstabilization (Mineyuki and Gunning 1990), it has been proposed thatmicrotubule reorganization is a key event in changing developmentalpatterns where altered division symmetry and cell wall dynamics definethe induced embryogenic structure (Simmonds 1994).

Spontaneous diploids have been reported to arise from anther culture ofbarley (Subrahmanyam and Kasha 1975), tobacco (Burk et al. 1972,Kasperbauer and Collins 1972;), corm (Ku et al. 1981) and B. napus(Charne et al. 1988). It has been hypothesized that diploids may occurthrough endomitosis, endoreduplication and/or nuclear fusion within thecell during early stages of culture (Sunderland et al. 1974, Keller andArmstrong 1978) and possibly from unreduced gametes (Wenzel et al. 1977,Chen and Beversdorf 1992). However, as the occurrence of spontaneousdiploids is an infrequent and inconsistent event, colchicine has beenused to increase the frequency. The techniques of colchicine applicationhas not changed much since Levan (1938) soaked onion roots in colchicinesolutions. Currently, apical meristems, secondary buds, tillers or rootsare treated with colchicine (see Wong 1989, Swanson 1990, Mathias andRobbelen 1991). Generally, about 50% of the treated plants areresponsive. These procedures are labor intensive (Chen and Beversdorf1992), hazardous (Depaepe et al. 1981, Hansen et al. 1988, Barnabas etal. 1991, Hassawi and Liang 1991) and costly (Hassawi and Liang 1991)because high concentrations of colchicine are needed. Furthermore, threemonths can be added to the plant regeneration time to recover homozygouslines (Beversdorf et al. 1987). Additional drawbacks to using thisapproach include the regeneration of chimeras (Hansen et al. 1988, Wanet al. 1989, Wong 1989, Swanson 1990, Bamabas et al. 1991), aneuploids(Zhao and Davidson 1984), abnormalities in plant development (Hart andSabnis 1976, Loh and Ingram 1983) and low seed yield. Application ofcolchicine to cultures prior to organ formation has producednon-chimeric doubled haploids from corn callus (Wan et al. 1989) andwheat anther culture (Barnabas et al. 1991). An effective alternative tocolchicine has not been reported to date but would be highly desirable(Wan et al. 1989, Hassawi and Liang 1991).

Trifluralin, a dinitroaniline herbicide (Probst et al. 1976), acts in amanner similar to colchicine, by disrupting spindle microtubules(Bartels and Hilton 1973). Trifluralin, unlike colchicine, has a higheraffinity for plant tubulin than for animal tubulin (Hess and Bayer 1977,Morejohn and Fosket 1984, Morejohn et al. 1984).

Non-chimeric doubled haploid plants were recovered from B. napus cv.Topas microspores cultured in the presence of colchicine or trifluralin,according to the present invention. These antimitotic agents wereapplied during the initial stages of culturing, while the microsporeswere undergoing the heat treatment (32.5° C.) used to induceembryogenesis. Trifluralin treated cultures generated normal embryoswhich germinated directly upon transfer to regeneration medium andproduced doubled haploid plants at frequencies approaching 60%. However,only about 20% of the plants recovered from colchicine treated cultureswere doubled haploids. Longer colchicine treatments resulted in higherfrequencies of fertile plants but embryo development was abnormal andseveral subcultures were required to induce plant development. Chen etal. (1994) also found that the cv. Topas responded negatively tocolchicine treatment, however other cultivars responded positively byincreasing the production of embryos and the frequency of fertileplants. However, it has been shown that colchicine can be used, insteadof heat, to induce embryogenesis from B. napus cv. Topas microspores.The embryos generated from colchicine-induced cultures, at non-inductivetemperatures were normal. Ninety percent of the plants recovered fromthese embryos were fertile.

Desirable genetic recombinants resulting from microsporogenesis can beexploited by recovering haploid plants from microspore derived embryos.The interesting recombinants can be used for the development of newvarieties or homozygous breeding lines. As haploid plants are sterile,the practical utilization of haploids in breeding programs relies on anefficient chromosome doubling technique to obtain fertile diploid plants(Subrahmanyam and Kasha 1975, Loh and Ingram 1983).

SUMMARY OF THE INVENTION

The purpose of this present invention was to demonstrate that microsporetreatment with anti-cytoskeleton agents or inhibitors of proteinsynthesis, at a non-inductive temperature, is sufficient to induceembryogenesis from plant microspores. This present invention shows thatcolchicine, a microtubule depolymerizing agent can induce microspores toundergo embryogenesis and that heat shock is not required. The presentinvention further shows that 90% of the plants regenerated from thecolchicine-induced embryos were doubled haploids.

The standard prior art methods of inducing embryogenesis rely onexposing the microspores to a high temperature, usually about 32.5° C.This method has been used to induce embryogenesis in a number of plantspecies, as fully discussed in the prior art. Haploid plants can readilybe regenerated from the embryogenic microspore cultures. However, thesehaploid plants are sterile and, therefore, not useful in genetic studiesand breeding programs. As discussed above, colchicine has been used withsome success to double the chromosome compliment in the cell and thusresults in doubled haploids.

The present invention is further directed to the use of trifluralin, adinitroaniline herbicide which acts in a manner similar to colchicine bydisrupting spindle microtubules. However, as described herewith, theapplication of colchicine or trifluralin during heat induction resultedin the recovery of a much higher frequency of doubled haploid plantsfrom the trifluralin treatment.

Thus, according to the present invention, there is provided a method ofinducing embryogenesis from plant microspores comprising:

treating a microspore plant culture with a sufficient amount of acytoskeleton inhibitor or a protein synthesis inhibitor at non-heatshock temperature; and

incubating for a sufficient time to induce embryogenesis.

According to the present invention there is provided a further method ofinducing embryogenesis and chromosome doubling from plant microsporescomprising:

treating a microspore plant culture of B. napus cv. Topas with 25 μM ofcolchicine at 25° C.;

incubating the microspore plant culture for 42 hours to induceembryogenesis and chromosome doubling;

reducing the colchicine to 12.5 μM by dilution; and

incubating at 25° C. for 3 to 4 weeks until embryos are formed.

Further, according to the present invention, there is provided a methodof producing doubled haploid plants from plant microspores comprising:

treating a microspore plant culture with a sufficient amount of amicrotubule inhibitor at a heat shock temperature for a sufficient timeto induce chromosome doubling.

In addition according to the present invention there is provided amethod of producing doubled haploid plants from plant microsporescomprising:

treating a microspore plant culture of B. napus with from 1 μM to 10 μMtrifluralin for 18 hours at 32.5° C. to induce chromosome doubling;

removing the trifluralin from the microspore culture and incubating at25° C. for 3 to 4 weeks until embryos are formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b show germinated embryos from colchicine-inducedmicrospore cultures (FIG. 1a) and heat-induced microspore cultures (FIG.1b).

FIGS. 2a and 2b show young plants from colchicine-induced microsporecultures (FIG. 2a) and heat-induced microspore cultures (FIG. 2b).

FIG. 3a and 3b show flowering plants from colchicine-induced microsporecultures (FIG. 3a) (fertile plants) and heat-induced microspore cultures(FIG. 3b).

FIGS. 4a and 4b show influoresences from colchicine-induced microsporecultures (FIG. 4a) (fertile plants) and heat-induced microspore cultures(FIG. 4b).

FIGS. 5a and 5b show mature plants from colchicine-induced microsporecultures (FIG. 5a) (fertile plants, pods shown at the arrows) andheat-induced microspore cultures (FIG. 5b).

FIG. 6 is a root tip cell showing the chromosome number of 2n=38. TheFeulgen stain was used on root tips of progeny from a plant regeneratedfrom microspore-derived embryos induced by colchicine treatment.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of this invention was to demonstrate that it is possible toinduce microspores to undergo embryogenesis with agents other than ahigh temperature treatment. In some cases the agent will also promotedoubling of the chromosome complement and thus generate doubled haploidplants.

The microspore cytoskeleton was modified with anti-microtubule oranti-microfilament agents at a non-heat shock temperature of about 25°C. Embryogenesis was induced by 18-50 hour treatments withanti-microtubule agents, namely, colchicine (10-100 uM), trifluralin(0.5-10 uM) or amiprophosmethyl (0.3-30 uM). Embryogenesis was alsoinduced by 5-60 min. treatments with the anti-microfilament agent,cytochalasin D (10-80 uM).

Microspore protein synthesis was inhibited with cycloheximide.Embryogenesis was induced by 7-24 h treatments with cycloheximide(0.03-3.5 uM) at the non-heat shock temperature of about 25° C.

Colchicine disrupts pollen development by depolymerizing microsporemicrotubules and may therefore be effective in inducing embryogenesis ina variety of species, including the more recalcitrant ones.

According to the present invention, microspores are incubated with from12.5 μM to 100 μM colchicine at non-heat shock temperatures, forexample, at about 25° C. for about 15 to about 50 hours. Following thisinitial treatment, the concentration of the colchicine is reduced andthe treated microspore cultures are further incubated at approximately25° C. until embryos form.

After the initial treatment of the microspore cultures with colchicine,the concentration of the colchicine is reduced. In one example of thepresent invention, the colchicine concentration is diluted by addingapproximately an equal volume of fresh medium which does not contain anycolchicine. In a further example of the present invention, thecolchicine is removed by washing the treated microspores andresuspending the washed microspores in fresh culture medium.

In one example of the present invention, the microspore culture istreated with 25 μM to 50 μM colchicine for 18 to 48 h at 25° C. In afurther example of the present invention, the microspores are culturedwith 25 μM colchicine at 25° C. for 42 h, followed by medium dilution.

Improved results were obtained when the microspore culture preparationswere staged for early development. It was found that either theunicellular-vacuolate (UV) stage or the late-unicellular (LV) stageresulted in the highest percentage of embryo generation. In these twostages, more than 78% of the microspores are unicellular microspores.This is in contrast to the known results for heat-treated microsporeswherein the higher frequency of embryo production is obtained frommature microspores (less than 75% unicellular).

The present invention is further directed to the use of colchicine ortrifluralin to generate doubled haploid plants, where heat treatment isused to induce embryogenesis. In this example of the invention, culturedmicrospores are incubated from approximately 30 to 35° C. in eithercolchicine at 12.5 to 25 μM or trifluralin at 1 to 10 μM for 0.5 to 18h. After this initial incubation period, the microspores are washed toremove the colchicine or trifluralin. The cultures are then incubated at25° C. from about 3 to 4 weeks until cotyledonary embryo development.

According to this aspect of the invention, it was found that theaddition of low concentrations of trifluralin (1 to 10 μM) toembryogenic microspore cultures provides a simple approach to doublingchromosome numbers to generate fertile double haploid plants. The use oftrifluralin is preferred to the use of colchicine. Chimeric plants areavoided because the chromosomes are doubled very early in culture. Themethod is simple, effective and inexpensive. Furthermore, trifluralin issafer to use than colchicine.

In an example of the present invention, cultured microspores areincubated at 32.5° C. in 1 μM to 10 μM trifluralin for 18 h. Themicrospores are then washed and incubation is continued at 25° C. untilembryos develop.

While this invention is described in detail with particular reference topreferred embodiments thereof, said embodiments are offered toillustrate but not limit the invention.

EXAMPLES Example 1 Induction of Embryogenesis with Colchicine

Donor plants and microspore cultures

The growth conditions for donor plants of Brassica napus cv. Topas line4079, and procedures for microspore developmental staging, isolation andculture were previously described (Telmer et al. 1992). For eachexperiment, buds were carefully selected for petal lengths ranging from2.4-2.7 and 2.7-3.1 mm to obtain microspore populations of class II(majority of cells at the UV stage) and class III majority of cells atthe LU, and/or mitotic (M) and bicellular (BC) stages!, respectively(Telmer et al. 1992).

Colchicine treatment

Colchicine (Sigma Chemical Co.) stock solutions of 5 mM in water werestored at 4° C. in darkness. When required, colchicine was diluted to 50and 100 μM in NLN-13 culture medium and filter sterilized (0.22 μmMillex-GS Millipore). Microspores were plated at a cell density of40,000 mL⁻¹ in 300 μL NLN-13 in Petri dishes (30×10 mm, Falcon 1008). Anequal volume of NLN-13 with 0, 50 or 100 μM colchicine was added. Foreach treatment at least 3 replicates were used and the dishes wereplaced in a plastic container at 100% humidity in an incubator at 25° C.for either 18 or 42 h in darkness.

Colchicine removal or dilution after 18 or 42 h of treatment

Coichicine was either diluted by adding an equal volume of fresh NLN-13,18 or 42 h after treatment or alternatively removed by washing. Twodifferent washing procedures were used: i. centrifugation/pelleting andii. collection with Spin-X tubes. In the centrifugation/pelletingprocedure two volumes of washing medium B5 (Gamborg et al. 1968)containing 0.35M mannitol! was added to the microspore cultures,transferred to sterile Eppendorf tubes and spun at 65 g for 3 min. Thepellets were resuspended in washing medium and the procedure wasrepeated two more times and then the pellets were resuspended in freshNLN- 13 medium and returned to culture dishes. To collect microsporeswith Spin-X tubes (2 mL, Microcentrifuge Filter Units, Durapore PVDF,membrane pore size 0.45 μm, Millipore), the cultures were transferred tothe Spin-X tube inserts and the medium was spun through at 40 g for 2min, while the cells remained on the surface of the Millipore membrane.Fresh NLN-13 medium was added to the surface to resuspend the cells andthis was also spun through. This procedure was repeated two more timesand the cells were resuspended in NLN-13 and cultured as previouslydescribed. Embryos were counted after 3-4 weeks of culture.

Controls

Each experiment included plated microspores cultured continuously at 25°C., and for 18 h (Table 1) or 24 h (Table 2 and 3) at 32.5° C. followedby 25° C. Controls were also washed or diluted to mimic colchicinetreatments. The frequency of embryogenesis did not differ significantlyin 18 and 24 h heat treatments (data not shown).

Induction of embryogenesis with colchicine: Concentration and durationof treatment

Microspores were treated with 25 or 50 μM colchicine for 18 or 42 h at25° C. The longer colchicine treatments (42 h) resulted in higherembryogenic frequencies at both concentrations of colchicine (Table 1).Although 50 μM colchicine was usually more effective than 25 μM intreatments in which washing (centrifugation/pelleting) was used toremove colchicine, the highest embryo frequency was obtained with a 25μM colchicine treatment of 42 h followed by medium dilution (Table 1).The lower concentration of 12.5 μM colchicine, induced embryogenesis ata much lower frequency (approx. 1.5%, data not shown).

                  TABLE 1    ______________________________________    The effects of colchicine concentration, duration of treatment,    and washing or dilution on embryogenesis frequencies from    microspores of B. napus cv. Topas as compared to    embryogenesis from heat-treated (32.5° C.) microspores..sup.a    Duration   Embryo frequency    of         Heat-treated                         C.sup.b H    Exper-          treatment                   % total         25 μM.sup.d                                           50 μM.sup.d    iment (h)      microspores.sup.c                             0 μM.sup.d                                   W.sup.e                                        D.sup.e                                             W    D    ______________________________________    A     18       26.0      0     0.12 .sup. NA.sup.f                                             0.22 NA                   (24.0-27.3)          42                       0.35 NA   0.36 NA    B     18       13.4      0     0.43 0.58 0.59 0.51                   (13.0-13.7)          42                       0.44 0.88 0.63 0.56    ______________________________________     .sup.a The results from two experiments are shown as means of at least     three replicates; other experiments show the same trends.     .sup.b Ratio of embryo frequency of colchicinetreated (washed or diluted     cultures) to heattreated controls (cultures not washed or diluted).     .sup.c Embryo frequency expressed as percent of total viable microspores     cultured. Figures show the average number of embryos of three replicates;     range is shown in brackets.     .sup.d Colchicine concentration.     .sup.e Microspores were either washed by centrifugation/pelleting (W) or     diluted (D) with an equal volume of fresh culture medium.     .sup.f NA--data not available.

Effects of washing or dilution of microspores on embryogenesis

Following colchicine treatment, the cells were either washed to removethe chemical or diluted to reduce its concentration. The washingprocedure using centrifugation/pelleting appeared to be detrimental toefficient embryo production. Cultures which were washed after a heattreatment had a significantly lower frequency of embryogenesis (8.7%)than heat-treated cultures, which were not washed (14.4%)(Table 2).Washing by collecting microspores on membranes of Spin-X tubes was moreeffective than the centrifugation/pelleting method but it was costly andlabor intensive (Table 2). However, the addition of an equal volume offresh culture medium to microspore cultures, following induction,resulted in the highest embryogenic frequencies and improved embryoquality (see below). Likewise, medium dilution following colchicinetreatment (25 μM) resulted in higher frequencies of embryogenesis thanwashing (Table 1).

                  TABLE 2    ______________________________________    Effect of washing or dilution of heat-induced (32.5° C.)    microspores of B. napus cv. Topas on embryogenesis frequencies.               Mean embryo frequency (%)                 No. of    Treatment    Experiments                            Control    Treated    ______________________________________    Washing:     17.sup.a   14.4(±0.7).sup.b                                       8.7(±0.8)*    centrifugation/pelleting    Washing: Spin-X-tubes                 5          13.1(±2.0)                                       11.4(±1.6)    Dilution.sup.c                 13         15.0(±1.1)                                       15.4(±0.9)    ______________________________________     .sup.a At least three replicates were used for each experiment.     .sup.b The figures show the mean value of the experiments; standard error     are shown in brackets.     *Significantly different from control, P < 0.01 (Ftest).     .sup.c The Petri dish contents were diluted with an equal volume of fresh     culture medium.

Microspore developmental stage responsive to coichicine

The range in frequency of microspore embryogenesis was examined usingthe optimized colchicine induction conditions of 25 μM for 42 h followedby medium dilution. The sixteen experiments conducted showed that thefrequency of embryo yield ranged from 0.1 to 15.2% (Table 3). Althoughthe induction frequency appears to be inconsistent, further analysisshows that microspore responsiveness to colchicine was developmentalstage specific. The data in Table 3 show that the microsporepreparations which were staged for early development, (predominantly UVand LU stages or more than 78% unicellular microspores, Experiments8-16), generated embryos at a mean frequency of 10.2% whereas only 1.6%embryos were generated from preparations staged for later development,(predominantly LU, M and BC stages or less than 75% unicellular,Experiments 1-7). Conversely, heat treatment produced a higher frequencyof embryogenesis (mean of 16.1%) from the more mature microsporepreparations than from the early developmental stages (mean of 8.8%).The ratios of C/H (colchicine induced/heat induced embryogenesis) alsoshow that the two induction methods affect different microsporedevelopmental stages.

                                      TABLE 3    __________________________________________________________________________    Frequencies of embryogenesis following heat (32.5° C.) or    colchicine (25 μM, 42 h, 25° C.)    induction of B. napus cv. Topas microspores cultured at various initial    developmental stages.    Initial Microspore Isolation                          Embryogenesis    % microspores at different                          % Total Microspores.sup.c    Exp.       developmental stages.sup.a                          Heat-  Colchicine-    no.       MU UV LU M  BC % UC.sup.b                          treated                                 treated                                        C.sup.d /H    __________________________________________________________________________    1  0   0  2  0 98  2  18.2   0.4    0.02    2  0  11 27 18 44 38  11.4   1.6    0.14    3  0  12 30 18 40 42  14.9   1.0    0.07    4  0  18 26 10 46 44  12.4   0.1    0.01    5  0  14 41 12 33 55  21.6   2.4    0.11    6  0  14 42 24 20 56  21.0   0.6    0.03    7  0  20 54  6 20 74  13.2   4.8    0.36                          X = 16.1(1.6).sup.e                                 X = 1.6(0.6)    8  8  18 53 15  6 79  13.4   11.8   0.88    9  2  31 49 12  6 82   9.0   8.3    0.92    10 10 54 24  0 12 88  14.0   13.6   0.97    11 8  49 40  1  2 97   7.6   6.1    0.80    12 20 50 27  2  1 97   7.6   8.5    1.12    13 8  53 37  2  0 98   9.0   11.0   1.22    14 5  66 29  0  0 100  9.5   15.2   1.60    15 25 64 11  0  0 100  6.1   11.7   1.92    16 34 36 29  0  0 100  3.1   6.1    1.97                          X = 8.8(1.1)                                 X = 10.2(1.1)    __________________________________________________________________________     .sup.a MU, midunicellular;     UV, unicellular vacuolate;     LU, late unicellular;     M, mitotic;     BC, bicellular.     .sup.b unicellular microspores, UC = MU + UV + LU.     .sup.c Embryo frequency of heattreated or colchicinetreated cultures     expressed as percent of total viable microspores cultured.     Colchicine treatments were diluted after 42 h of culture. Each figure     represents the mean embryo frequency of at least three samples.     .sup.d Ratio of embryo frequency of colchicinetreated (C) microspores to     heattreated (H) microspores.     .sup.e S.E. is shown in brackets.

Quality of colchicine-induced embryos

The quality and yield of embryos in cultures induced with 25 μMcolchicine and diluted after 42 h of treatment was superior to cultureswashed after treatment or treated continuously with 25 μM colchicine butcomparable to the heat-induced cultures which were diluted after 42 h ofculturing. It is noteworthy that the embryo quality of the heat-inducedcultures was improved by dilution with fresh culture medium. The rate ofdevelopment of colchicine-induced microspores was initially slower by3-4 days than that of the heat-induced microspores. However, there wasno obvious difference in the size of embryos induced by these twodifferent treatments after 4 weeks of culture. A swelling of thehypocotyl region in some embryos induced by colchicine was observed, butthis did not affect embryo germination. The embryos generated from 50 μMcolchicine treatments followed by medium dilution formed large globularstructures and developed abnormally.

The significant finding in this invention is that colchicine can induceembryogenesis from microspores of B. napus cv. Topas at thenon-inductive temperature of 25° C. Therefore a heat shock is notrequired to induce embryogenesis. Colchicine-induced microtubuledepolymerization can change microspore development from gametogenesis toembryogenesis provided that the microspore isolate is predominantly UVand LU, or more specifically, more than 78% unicellular. Colchicinespecificity for the unicellular microspore is most probably a result ofmicrotubule susceptibility to colchicine at the unicellular stage;another study reported that colchicine treatment resulted in completedepolymerization of unicellular microtubules whereas the microtubules ofbicellular microspores were almost unaffected.

Heat induction is optimal in microspore isolates containing moreadvanced developmental stages, namely LU, M and BC (less than 75%unicellular) or Class III (Telmer et al. 1992). The two inducing agents,heat and colchicine, appear to act on different microspore developmentalstages, however this is partly due to the fact that they requiredifferent periods of time to exert their effects. Nuclear migration awayfrom the edge of the cell and the appearance of PPBs after about 6 h ofheat treatment indicate that morphological changes including microtubulereorganization occur very rapidly with heat induction (Simmonds 1994;Telmer et al. 1995). Colchicine induction requires a longer period oftime. Cell growth and development is slower at 25° C. than at 32.5° C.Microtubule depolymerization by colchicine is a slow process, requiringup to 8 h. Following microtubule depolymerization, the nucleus migratesaway from the edge of the cell. This indicates that microtubules have arole in nuclear anchoring and maintaining cell asymmetry in pollendevelopment (Simmonds 1994). During the time period needed formicrotubule depolymerization, microspore development continues and someof the microspores which began as UV or LU, at culture initiation,arrive at mitosis with depolymerized microtubules and a centrallylocated nucleus, primed to undergo a symmetrical division. Therefore,the LU microspore which enters mitosis with altered morphology remainsthe competent or inducible stage for both inducing agents. However, theLU microspores which enter mitosis during the initial phase of culturingescape the effects of colchicine and divide asymmetrically, as pollen;as these bicellular microspores are insensitive to colchicine, theycontinue normal pollen development in its presence. However, it has beenshown that a high frequency of maturing bicellular microspores inhibitthe development of induced microspores, possibly by releasing someinhibitory factor(s) (Kott et al. 1988; Simmonds et al. 1991).Therefore, the requirement of predominantly unicellular microspores forcolchicine induction may be due to a combination of factors, includingmicrospore susceptibility to colchicine at the unicellular stage, slowmicrotubule depolymerization, relatively slow cell development at 25° C.and embryogenic inhibitory properties of maturing bicellularmicrospores.

As colchicine induction and heat induction target different initialmicrospore populations, it seems reasonable to expect that thecombination of the two treatments would result in a greater yield ofembryos than the individual treatments. However, simultaneous heat andcolchicine treatments was reported to reduce embryogenesis in B. napuscv. Topas (Chen et al. 1994). Contrary to these results, an increase inembryogenesis was reported by Zaki and Dickinson (1991) and Iqbal et al.(1994). As these workers did not stage their microspore populations,this combination of treatments may be beneficial when heterogeneouspopulations of microspores are cultured or when embryogenesis is low. Itmay not enhance embryogenesis when microspore isolations are screenedfor optimal developmental stages and very high embryogenic frequency hasbeen attained with heat induction alone.

The appearance of PPBs and the change in microtubule organization duringheat induction may be the result of heat shock. All organisms that havebeen examined to date respond to heat shock by synthesizing heat shockproteins and simultaneously inhibiting the synthesis of housekeepingproteins (Vierling 1991). While heat shock proteins have been proposedto be involved in the induction of B. napus cv. Topas microsporeembryogenesis (Pechan et al. 1991; Cordewener et al. 1994) it ispossible that the inhibition of synthesis of certain pollen-specificproteins which may be concomitant with heat shock could be moreimportant. Such proteins may participate in maintaining morphologicalasymmetry and their removal would lead to a loss in cell asymmetry asindicated by microtubule reorganization and nuclear migration. Once thisasymmetry is lost it may not be possible to re-establish it. Thusembryogenesis may occur as a default mechanism as previously suggested(Telmer et al. 1992, 1994; Simmonds 1994). Colchicine binds to α and βtubulin heterodimers which inhibits further dimer addition tomicrotubules and results in eventual microtubule depolymerization (Hartand Sabins 1976; Margolis and Wilson 1977; Sternlicht et al. 1983). Theelevated concentration of free tubulins acts to depress the synthesis ofnew α and β tubulins (Cleveland et al. 1983; Lau et al. 1985). Theincrease in concentration of free tubulins may also depress thesynthesis of pollen specific tubulins (Carpenter et al. 1992) and thusprevent the progression of pollen development. However, the primaryaction of colchicine, microtubule depolymerization, releases theanchored nucleus thus disrupting microspore asymmetry (Simmonds 1994).Although heat induction and colchicine induction of microspores may actoptimally at different points in microspore development they both appearto function through a default mechanism, by reorganizing thecytoskeleton, which leads to a loss of cell asymmetry and blocks pollendevelopment.

Thus, colchicine can be used to induce microspore embryogenesis in B.napus cv. Topas. Colchicine disrupts pollen development bydepolymerizing microspore microtubules and may therefore be effective ininducing embryogenesis in a variety of species including the morerecalcitrant ones. Colchicine induction of embryogenesis adds anotherdimension in the study of induction processes; by comparing colchicine-and heat-induced embryogenesis it will be possible to distinguishfactors specific to embryogenesis and those specific to heat-induction.Another advantage in using colchicine is that it is also a chromosomedoubling agent and over 90% of the plants regenerated from thecolchicine-induced embryos were found to be doubled haploids. A simpleone step process to simultaneously induce embryogenesis and chromosomedoubling for the production of fertile plants is very advantageous forgenetic studies and plant breeding programs.

Microspore Embryogenesis

Plant growth conditions for B. napus cv. Topas, line 4079 and theprocedures for microspore isolation, culture and induction ofembryogenesis with a heat treatment (32.5°, 24 h) was previouslydescribed (Telmer et al. 1992). Microspore embryogenesis was induced atthe non-inductive temperature of 25° C. by treating microspore cultureswith 25 μM colchicine for 42 h followed by dilution to 12.5 μM withfresh culture medium as described above.

Plant regeneration

Cotyledonary embryos derived from both heat (32.5° C.) and colchicine(25° C.) treated microspores were subcultured onto solid B5 medium(Gamborg et al. 1968) containing 2% sucrose and 0.2% gelrite (Kelco,Division of Merck and Co. Inc., San Diego, Calif.), and cultured at 20°C. at a 16 h photoperiod illumination provided by incandescent andfluorescent lights (90 μmol⁻² s⁻¹)!. Plantlets at the 3 leaf stage weretransferred to soil (3 cm pots) and were maintained in a mist chamberfor 2 weeks. They were then repotted in 15 cm pots and grown to maturityin the greenhouse at 20/15° C. day/night at a photoperiod of 16 h andlight intensity of 280 μmol⁻² s⁻¹.

Determination of fertility

Evaluation of fertility for all regenerated plants was based on bud andflower size, pollen production and seed set. Plants producing pollenwere bagged for self pollination

Cytological analysis

Chromosome numbers of the progeny of fertile plants was determined aspreviously described.

Plantlet regeneration and development

Cotyledonary embryos derived from heat- and colchicine-treatedmicrospores were subcultured onto solid regeneration medium. The embryosgerminated and developed to the three leaf stage within three weeks in32 of 80 and 59 of 120 heat-induced and colchicine-induced embryos,respectively (FIG. 1, Table 4). Plants were regenerated from theseplantlets (FIG. 2). Lateral branching and leaf sizes were similar in allthe regenerated plants (FIG. 3). However, 90% of the colchicine-derivedplants produced much larger buds and flowers than the heat-derivedplants (FIG. 4.). These colchicine-derived plants produced pollen, werefertile and had normal seed set (FIG. 5, Table 4); 94% of theheat-derived plants had vestigial anthers, no pollen and were sterile(FIG. 6, Table 4). Neither group produced any sectored plants.

                  TABLE 4    ______________________________________    Plant regeneration, fertility and ploidy levels from heat induced    and colchicine induced embryos of Brassica napus cv. Topas.    Mode of    induction of                Embryos  Plants    Fertile                                          Ploidy    embryogenesis                cultured regenerated                                   Plants level    ______________________________________    Colchicine (25° C.)                120      59        53     2n = 38.sup.a    Heat (32.5° C.)                80       32        2      2n = 38    ______________________________________     .sup.a Ploidy levels were determined from germinated seeds of 30 randomly     selected plants, and 2 fertile plants from the colchicine and heat induce     embryogenesis, respectively. They all showed 2n = 38 chromosomes.

Cytological analysis

The ploidy levels of the progeny of fertile plants derived from bothheat- and colchicine-induced embryos was examined cytologically. Seedswere germinated from the 2 plants derived from heat induction and 30randomly selected plants derived from coichicine induction. All of theprogeny showed 2n=38 chromosomes (FIG. 6).

Thus, the microtubule depolymerizing agent, colchicine, used to induceembryogenesis in microspores can simultaneously double the ploidy levelof the haploid cells. Sectored chimeras were not produced. The procedureis very simple, normal embryos are generated and fertile plants areregenerated rapidly. There are many advantages implicit in thisprocedure. Field trials and seed analysis can begin immediately becauseseed yield is high. The elimination of an additional growth cyclereduces cost and speeds up variety development. It is not necessary tomaintain as many plants to find doubled haploids as most will befertile. The use of coichicine at low concentration and low volume,reduces cost and toxity problems.

Ninety percent of plants recovered from the colchicine induced cultures,compared with 6% for heat treated cultures, developed large buds andflowers, produced abundant pollen, and set seed. The remaining plants(10% from the colchicine treatments and 94% from the heat treatments)had small buds and flowers, no pollen and did not set seed. Thisconfirms studies (e.g. Keller and Armstrong 1978) which suggested thatboth bud and flower size can be used to discriminate between haploid anddiploid plants. Cytological analysis of the progeny showed stable ploidylevels of 2n=38.

In addition to the genome doubling efficiency described, no chimeras orsectored plants were produced with the colchicine induction method. Thefundamental difference between this method and others for production ofdoubled haploid plants is that unicellular microspores (Zhao et al.1995), rather than multicellular organs were used as the target of thecolchicine treatment. Chromosome doubling in the colchicine-treatedcultures most likely occurred during the first microspore mitosis atwhich time spindles would not have formed in the presence of colchicinethus disabling chromosome segregation. Nevertheless, it is known thatnuclear restitution follows, irrespective of the chromosome arrangement(e.g. Zhao and Davidson 1984). The next mitotic cycle most probablyoccurred after colchicine dilution, but with 2n chromosomes. Very earlydoubling of chromosomes could account for the absence of chimerasamongst the fertile doubled haploid plants recovered.

Approximately 50% of embryos derived from colchicine-induced microsporesdeveloped directly into normal plants. Abnormal embryo developmentfollowing colchicine treatment has been observed in B. napus, B. napusssp. oleifera (Loh and Ingram 1983) and in Triticum aestivum (Hansen etal. 1988). However, in the results described above, abnormal embryoswere derived from microspores treated with heat to induce embryogenesisand cultured with colchicine to promote chromosome doubling; high levelsof colchicine or the continuous presence of low levels undoubtedlyaffect embryo development (Zhao and Simmonds 1994). According to theresults of the present invention, normal embryo development was seenfollowing colchicine induction without the heat treatment. Induction ofembryogenesis with colchicine was achieved with earlier microsporedevelopmental stages than the optimum stages for heat induction and thecolchicine was diluted after induction (Zhao et al. 1994). Thisindicates that heat and colchicine induction are effective on differentmicrospore developmental stages and the removal or dilution ofcolchicine is important for normal embryo development. The combinationtreatment of colchicine plus heat followed by washing has beensuccessfully employed to obtain doubled haploids in several B. napuscvs. and breeding lines but not in the cv. Topas used herein (Chen etal. 1994, Mollers et al. 1994). It is possible that the combinationtreatment is effective when a broad range of microspore developmentalstages are cultured and when the frequency of embryogenesis is low.

The use of colchicine to induce embryogenesis in microspore cultures andsimultaneously double the chromosomes offers a potentially powerful toolfor producing pure breeding lines of B. napus at high frequencies, ahighly desirable combination for plant breeding programs. Furthermore,as all eukaryotic cells are affected similarly by colchicine, i.ethrough microtubule depolymerization, it is possible that this techniquewill be applicable to other species, particularly the more recalcitrantones.

Example 2 Generation of Doubled Haploid Plants with TrifluralinMicrospore isolation and culture

B. napus cv. Topas, line 4079, was grown as described previously (Telmeret al. 1992). Buds were scored for the stage of microspore developmentand microspores near the first pollen mitosis were isolated and culturedin NLN-13 culture medium (Telmer et al. 1992). Microspore density wasadjusted to 40,000 ml⁻¹ NLN-13 medium, and 300 μl were plated in 10×30mm Petri dishes (Falcon 1008, VWR Scientific of Canada, Toronto). Anadditional 300 μl of NLN-13 medium was added to the control plates and300 μl of NLN-13 containing either colchicine or trifluralin at 2× thefinal concentration was added to the experimental plates. The cultureswere incubated in a humid chamber in darkness, for 18 h at 32.5° C.After 18 h at 32.5° C. the cultures were moved to 25° C. where theyremained in darkness for 3-4 weeks until cotyledonary embryos developed.When required, the microspores were washed after 18 h of heat treatmentto remove colchicine or trifluralin. The contents of each Petri dish wasdiluted with 1 ml B5 (Gamborg et al. 1968) containing 0.4M mannitol andwashed 3× in Eppendorf tubes by centrifugation (100 g, 3 min). Afterwashing, the cells were resuspended in 600 μl of NLN-13 and cultured at25° C.

Colchicine and trifluralin stock solutions

Stock solutions of 5 mM colchicine (Sigma, St. Louis, Mo.) and 1 mMtrifluralin (α,α,α-trifluoro-2,6-dinitro-N-N-dipropyl-p-toluidine) (EliLilly and Co., Indianapolis, Ind.) were prepared in double distilledwater and in acetone, respectively, and were stored in darkness at 6-9°C. The chemicals were diluted to the required concentration in NLN-13and filter sterilized prior to use.

Embryo germination and plant regeneration

Cotyledonary embryos were transferred onto solid B5 medium containing 2%sucrose and 0.2% gelrite (Kelco, Merck & Co. Inc., San Diego, Calif.)and cultured at 20° C. and a 16 h photoperiod provided by 115 W CoolWhite fluorescent lights (Sylvania) and 40 W incandescent lights DuroTest (90 μmolm⁻² s⁻¹)!. The majority of the embryos germinated within2-3 weeks. Embryos which did not form roots directly were subculturedone or more times on the same medium to induce root development.Plantlets, at the three leaf stage were transferred to soil andvermiculite (1:1) in 1.25 inch pots and grown in a mist chamber forabout 2 weeks. They were then repotted in soil in 6 inch pots and grownin the greenhouse at 20/15° C. day/night with a photoperiod of 16 h andsupplemented with High Pressure Sodium lucolux lights (General Electric)producing a light intensity of 280 μmolm⁻² s⁻¹. Plants with viablepollen were self-pollinated by bagging racemes with unopened buds.

Cytological analysis

Seeds harvested from the selfed plants were germinated on moistenedfilter paper at 25° C. in darkness and roots were collected 48 h aftergermination. Roots were placed in ice-water for 3 h followed by a 4 htreatment in 2 mM 8-hydroxyquinoline at 15° C. (Newell et al. 1984) anda 24 h fixation in absolute ethanol:glacial acetic acid (3:1) at roomtemperature; they were stored in 70% ethanol at 4° C. To stainchromosomes, roots were hydrolysed in 1N HCl at 60° C. for 7 min,stained in Feulgen solution for 1 h followed by 30 min in ice-water andsoftened with 1% pectinase at room temperature for 5 min. Root meristemswere squashed on a slide in 1% aectocarmine and 45% acetic acid andviewed on a Zeiss Axiophot microscope. Technical Pan film (Kodak,Canada) was used for photography.

Microtubule labelling

Microtubules were visualized with indirect immunofluorescence labellingof microspores which were fixed immediately after isolation, and after0.5, 3, 8, and 18 h of culture. The cells were simultaneously labelledfor chromatin with Hoechst 33258 and for microtubules using the primarymonoclonal rat anti-yeast tubulin (MAS 078, clone YOL 1/34, CedarlaneLaboratories, Hornby, Ont. Canada) followed by the secondary antibody,fluorescein-conjugated goat anti-rat immunoglobulin IgG (Sigma)(Simmonds et al. 1985; Simmonds and Setterfield 1986). Samples wereviewed on a Zeiss Photomicroscope III equipped with epifluorescenceoptics using filter sets 10(BP450-490+BP520-560) and 02(G365+LP420) forfluorescein and Hoechst fluorescence, respectively. At least 300microspores were examined per sample and Ilford XPI-400 film was usedfor photography.

Depolymerization of microspore microtubules with trifluralin orcolchicine

Microspores were cultured in the presence of 0.1, 1.0 and 10 μMtrifluralin and 2.5 and 25 mM colchicine and examined after 0.5, 3, 8and 18 h of culture. Untreated uninucleate microspores show abundantmicrotubules radiating from the nuclear envelope. At a concentration of0.1 μM, trifluralin did not depolymerize microtubules. However, after 30min of either 1.0 or 10 μM trifluralin treatments, all microtubules inuninucleate microspores were depolymerized and at 18 h, microtubulerecovery was observed in more than 50% of the microspores in the 1.0 μMtreatment and approx. 30% of the microspores in the 10 μM treatment. Themicrotubules did not reorganize into the type of arrays seen prior tothe treatment but formed one or several, long, randomly oriented strands(data not shown). Microtubules were not completely depolymerized after 3h of 25 μM colchicine treatment but at 8 and 18 h they were no longervisible; 2.5 μM colchicine did not depolymerize microspore microtubules.At the concentrations tested, neither chemical was effective indepolymerizing microtubules in bicellular microspores; some microtubuledepolymerization and disorganization was observed but completedepolymerization was not obtained.

Embryo development

The effect of culturing microspores in the presence of colchicine ortrifluralin on embryo frequency and quality is shown in Table 5. Theembryo frequencies from microspore cultures treated with the chemicalswere lower than the controls. However, the 18 h treatments at the lowerconcentrations of colchicine and trifluralin produced more embryos thanthe cultures subjected to continuous treatments and the higher chemicalconcentrations. Furthermore, the embryos generated from 18 h treatmentsdeveloped normally whereas abnormal development was prevalent in thecontinuous treatments with 10 μM trifluralin, and 25 and 50 μMcolchicine. The embryos classified as abnormal had enlarged cotyledonsand swollen hypocotyls or continued enlarging as globular forms butnever developed cotyledons.

                  TABLE 5    ______________________________________    Influence of colchicine or trifluralin treatments    (18 h or continuous) on the frequency and quality of    embryos derived from heat-induced (32.5° C. for    18 h) B. Napus cv. Topas microspores.                 No. of    T.sup.b   Embryo    Treatment.sup.a                 experiments                           C         development    ______________________________________    18 h Treatments    Control/washed                 9         1.0       normal    1 μM      4         0.71 ± 0.11                                     normal    trifluralin/washed    10 μM     2         0.56      normal    trifluralin/washed    25 μM     6         0.89 ± 0.03                                     normal    colchicine/washed    50 μM     2         0.64      normal    colchicine/washed    Continuous Treatments    Control      9         1.0       normal    1 μM      4         0.28 ± 0.1                                     normal    trifluralin/cont.    10 μM     2         0.06      abnormal    trifluralin/cont.    25 μM     6         0.39 ± 0.01                                     abnormal    colchicine/cont.    50 μM     2         0.02      abnormal    colchicine/cont.    ______________________________________     .sup.a Microspore cultures were either treated for 18 h and then washed o     were cultured continuously (cont.) in the presence of colchicine or     trifluralin as described in materials and methods.     .sup.b Ratio (±SE) of embryo frequency for treated (T) to control (C)     embryo frequency for the respective washed or continuous cultures. Actual     frequencies of embryogenesis for control/washed and control cultures were     7.4 ± 1.4 and 12 ± 2.0, respectively, and represent the frequency a     the percent of total viable microspores.

Influence of colchicine or trifluralin on the generation of fertileplants

More than 85% of the plants regenerated from untreated embryogeniccultures were sterile as they lacked pollen and did not produce seed(Table 6). Cultures treated continuously with 25 μM colchicine producedfertile plants at a frequency of about 50% (Table 6). However, as theembryos were abnormal (Table 5), even the best of these embryos did notgerminate directly and required several subcultures on B5 medium forplantlet development. Plants generated from this treatment were lessvigorous and less uniform than those produced from other treatments.When the cultures were washed after 18 h of colchicine treatment, embryoand plant development were normal but only 22% of the regenerated plantswere fertile (Tables 5 and 6).

                  TABLE 6    ______________________________________    Fertility of B. napus cv. Topas plants recovered from    microspore cultures heat-induced (32.5° C. for 18 h)    in the presence of trifluralin or colchicine.                           No.                 Duration.sup.a                           fertile plants                                     %    Treatment    of Treatment                           total plants                                     fertile plants    ______________________________________    Control                6/50      12    25 μM colchicine                 18 h      6/27      22    25 μM colchicine                 cont.     9/17      53    1 μM trifluralin                 18 h      14/25     56    10 μM trifluralin                 18 h      7/12      58    1 μM trifluralin                 cont.     15/77     20    10 μM trifluralin                 cont.     0/0       NA    ______________________________________     .sup.a Microspore cultures were either treated for 18 h and then washed o     were cultured continuously (cont.) in the presence of colchicine or     trifluralin as described in materials and methods.

Cultures treated with 1 or 10 μM trifluralin for 18 h generated fertileplants at frequencies approaching 60% (Table 6). The embryos derivedfrom these cultures were normal, germinated readily when transferred toB5 medium and produced vigorous plants. Microspores culturedcontinuously with 1 μM trifluralin generated fewer fertile plantswhereas no plants were regenerated from the 10 μM treatments due to thelow embryo frequency and abnormal embryo development (Table 5 and 6).

Cytological analysis of progeny from the fertile plants

Cytological studies were carried out on the progeny of approximately 50%of the fertile plants from each treatment and confirmed the ploidy levelto be 2n=38 chromosomes. The chemically induced doubled haploid plantsproduced seed on all the branches which indicates that they were notchimeric. The fertile plants derived from microspore cultures which hadnot been treated with colchicine or trifluralin showed the same ploidylevel and were not chimeric.

This is the first study to show that trifluralin is an effectivechromosome doubling agent and can be used to produce doubled haploidplants of B. napus. Fertile plants were obtained from nearly 60% of theplants derived from microspores treated with 1 or 10 μM trifluralin for18 h. However, 1 μM trifluralin was the better concentration becauseembryo yield was higher. A great advantage in using trifluralin is thatembryogenesis is normal and proceeds to direct embryo germination andvigorous plant growth.

Colchicine treatments of 18 h produced fertile plants at a frequency ofonly 22%. It is possible that a colchicine treatment of 18 h is tooshort because it appears to be a slower acting drug than trifluralin asindicated by the 3-8 h required to depolymerize microtubules as comparedto 30 min with trifluralin. Other workers have shown that longertreatments with coichicine increased the frequency of chromosomedoubling in corn callus (Wan et al. 1989) and the continuous colchicinetreatment here produced more than 50% fertile plants. Unfortunately,continuous treatment with colchicine resulted in the production ofabnormal embryos requiring time-consuming subcultures and ultimatelypoor plant development. Similarly, continuous exposure to trifluralinresulted in reduced embryo yields and at a concentration of 10 μMtrifluralin resulted in abnormal embryo development. After prolongedexposure to microtubule depolymerizing agents, microtubules recover andplant cells resume normal division and growth; microtubules werebeginning to reappear after 18 h of culture in the presence oftrifluralin. However, it is unlikely that normal spatial and temporalorganization of the cytoskeleton would recover rapidly enough to besynchronized and functional with the next cell cycle. This would impedenormal development, as was observed in this and other studies (Hart andSabnis 1976, Loh and Ingram 1983, Hansen et al. 1988, Mathias andRobbelen 1991). Microtubule recovery is slower at higher concentrationsof the depolymerizing agents which would further reduce embryogenicfrequency (Table 5).

The generation of fertile non-chimeric plants is most probably aconsequence of application of microtubule depolymerizing agents toselected Class III microspore isolations which contained large numbersof late uninucleate microspores (Telmer et al. 1992). Microspores entermitosis within the first few hours of culture, and by 12 h, the majorityhad undergone division (Telmer et al. 1994). In the presence ofmicrotubule depolymerizing agents, mitosis proceeds through this stagewithout a spindle which blocks chromosome segregation but nuclearrestitution and the cell cycle proceed normally, except that thechromosome number is doubled (Lignowski and Scott 1972, Zhao andDavidson 1984). The microspores which pass through mitosis early in theculture period would most probably undergo chromosome doubling in thepresence of trifluralin which depolymerizes microtubules rapidly.Colchicine, on the other hand requires 3-8 h to depolymerizemicrotubules and would not affect the early microspore divisions. Thismay explain why more fertile plants were obtained from cultures treatedfor 18 h with trifluralin than with colchicine. Furthermore, a highfrequency of fertile plants were obtained from the continuous colchicinetreatments indicating that the chromosome doubling probably occurredduring the second microspore division. It is possible that themicrospores subjected to continuous 1, μM trifluralin treatmentsunderwent one cycle of chromosome doubling during the first few hours ofculture and a second cycle of doubling prior to recovery of microtubulefunction. Such polyploid microspores, if they were produced must haveceased further growth because polyploid plants were not recovered. Thismay explain the reduced embryo frequency and the low frequency offertile plants obtained from the 1 μM continuous trifluralin treatment.

In conclusion, the addition of a very low concentration of trifluralinto embryogenic microspore cultures of B. napus offers a very simpleapproach to doubling chromosome numbers to generate fertile doubledhaploid plants. Microspore embryogenesis is normal and leads to directgermination and development of vigorous plants. It is a time and laborsaving alternative to the application of colchicine to plant tissues.Chimeric plants are avoided because chromosomes are doubled very earlyin culture. The method is simple, effective and inexpensive. Trifluralinhas a much higher affinity for plant cells than animal cells and at theconcentration used has no effect on animal cells (Bartels and Hilton1973, Bayer et al. 1967, Hess and Bayer 1977) therefore making it muchsafer to use than colchicine. As trifluralin has been shown todepolymerize microtubules in diverse plant tissues (Hess and Bayer 1977,Hess 1979, Quader and Filner 1980), it may be equally as effective, as adoubling agent, in other plant species.

All scientific publications and patent documents are incorporated hereinby reference.

The present invention has been described with regard to preferredembodiments. However, it will be obvious to persons skilled in the artthat a number of variations and modifications can be made withoutdeparting form the scope of the invention as described in the followingclaims.

References

Bamabas, B., P. L. Pfahler, and G. Kovacs, 1991: Direct colchicine onthe microspore embryogenesis to produce dihaploid plants in wheat(Triticum aestivum L.). Theor. Appl. Genet. 81, 675-678.

Barnabas, B., Pfahler, P. L. & Kovacs, G. 1991. Direct effect ofcolchicine on the microspore embryogenesis to produce dihaploid plantsin wheat (Triticum aestivum L).--Theor. Appl. Genet. 81:675-678.

Bartels, P. G. & Hilton, J. L. 1973. Comparison of trifluralin,oryzalin, pronamide, propham and colchicine treatments onmicrotubules.--Pesticide Biochem. Physiol. 3:462-472.

Bayer, D. E., Foy, C. L., Mallory, T. E. & Cutter, E. G. 1967.Morphological and histological effects of trifluralin on rootdevelopment.--Am. J. Bot. 54:945-952.

Beversdorf, W. D., Charne D. G., Kott, L. S., Chuong, P. V., Polsoni, L.& Zilka, J. 1987. The utilization of microspore culture andmicrospore-derived doubled-haploids in a rapeseed (Brassica napus)breeding program.--In Proc. 7th Int. Rapeseed Conf, (OrganizingCommittee, ed), pp. 13. Poznan, Poland.

Burk, L. G., G. R. Gwynn, G. F. Chapline, 1972: Diploidized haploidsfrom aseptically cultured anthers of Nicotiana tabacum: A colchicinemethod applicable to plant breeding. J. Hered. 63, 355-360.

Carpenter, J. L., Ploense, S. E., Snustad, D. P., Silflow, C. D. (1992)Preferential expression of an α-tubulin gene of Arabidopsis in pollen.Plant Cell 4, 557-571.

Charne, D. G., P. Pukacki, L. S. Kott, W. D. Beversdorf, 1988:Embryogenesis following cryopreservation in isolated microspores ofrapeseed (Brassica napus L.). Plant Cell Rep. 7, 407-409.

Gamborg, O. L., Miller, R. A., Ojima, J. (1968) Nutrient requirements ofsuspension cultures of soybean root cells. Exp. Cell Res. 50, 151-158.

Gland, A., Lichter, R., Schweiger, H. G. (1988) Genetic and exogenousfactors affecting embryogenesis in isolated microspore cultures ofBrassica napus L. J. Plant Physiol. 132, 613-617.

Gunning, B. E. S., Hardham, A. R. (1982) Microtubules. Annu. Rev. PlantPhysiol. 33, 651-698.

Hansen, F. L., I. K. D. Andersen, and A. Olesen, 1988: Nitrous oxide asa possible alternative agent for chromosome doubling of wheat haploids.Plant Sci. 54, 219-222.

Hart, J. W. & Sabnis, D. D. 1976. Colchicine and plant microtubules: Acritical evaluation.--In Commentaries in Plant Sci. (H. Smith, ed), pp.1095-1104. Pergamon Press, Oxford, N.Y. ISBN 0-080-19759-0.

Hassawi, D. S. & Liang, G. H. (1991) Antimitotic agents: Effects ondoubled haploid production in wheat. Crop Sci. 31:723-726.

Hess, F. D. & Bayer, D. E. 1977. Binding of the herbicide trifluralin toChlamydomonas flagellar tubulin.--J. Cell Sci. 24:351-360.

Hess, F. D. 1979. The influence of herbicide trifluralin on flagellarregeneration in Chlamydomonas.--Exp. Cell Res. 119:99-109.

Huang, B., Bird, S., Kemble, R., Simmonds, D., Keller, W., Miki, B.(1990) Effects of culture density, conditioned medium and feedercultures on microspore embryogenesis in Brassica napus L. cv. Topas.Plant Cell Rep. 8, 594-597.

Chen, J. L., Beversdorf, W. D. (1992) Production of spontaneous diploidlines from isolated microspores following cryopreservation in springRapeseed (Brassica napus L.). Plant Breeding 108, 324-327.

Chen, Z. Z., Snyder, Z., Fan, Z. G., Loh, W. H. (1994) Efficientproduction of doubled haploid plants through chromosome doubling ofisolated microspores in Brassica napus. Plant Breeding 113, 217-221.

Chuong, P. V., Beversdorf, W. D. (1985) High frequency embryogenesisthrough isolated microspore culture in Brassica napus L and B. carinataBraun. Plant Sci. 39, 219-226.

Cleveland, D. W., Pittenger, M. F., Feramisco, J. R. (1983) Elevation oftubulin levels by microinjection suppresses new tubulin synthesis.Nature 305, 738-740.

Collins, G. B. & Genovesi, A. D. 1982. Anther culture and itsapplication to crop improvement. In Application of plant cell and tissueculture to agriculture and industry. (D. T. Tomas, B. E. Ellis, K. J.Kasha and R. L. Peterson, eds), pp. 1-24. Univ Guelph.

Cordewener, J. H. G., Busink, R., Traas, J. A., Custers, J. B. M., Dons,H. J. M., Van Lookeren Campagne, M. M. (1994) Induction of microsporeembryogenesis in Brassica napus L. is accompanied by specific changes inprotein synthesis. Planta 19 195, 50-56.

Depaepe, R., Bleton, D. & Gnangbe, F. 1981. Basis and extent of geneticvariability among doubled haploid plants obtained by pollen culture inNicotiana sylvestris.--Theor. Appl. Genet. 59: 177-184.

Fan, Z., Armstrong, K. C., Keller, W. A. (1988) Development ofmicrospores in vivo and in vitro in Brassica napus L. Protoplasma 147,191-199.

Huang, B. (1992) Genetic manipulation of microspores andmicrospore-derived embryos. In Vitro Cell Dev. Biol. 28, 53-58.

Iqbal, M. C. M., Mollers, C., Robbelen, G. (1994) Increasedembryogenesis after colchicine treatment of microspore cultures ofBrassica napus L. J. Plant Physiol. 143, 222-226.

Kasperbauer, M. J. & Collins, G. B. 1972. Reconstitution of diploidsfrom leaf tissue of anther-derived haploids in tobacco.--Crop Sci.12:98-101.

Keller, W. A., Arnisn, P. G., Cardy, B. J. (1986) Haploids fromgametophytic cells--recent developments and future prospects. In: PlantTissue and Cell Culture. Proc. 6th Int. Tissue Culture Congr. pp.223-241, Green, C. E., Somers, D. A., Hacoett, W. P., Biesboer, D. D.,eds. Alan R Liss Inc., New York, N.Y. ISBN 0-8-45118-021.

Keller. W. A., Armstrong, K. C. (1978) High frequency production ofmicrospore-derived plants from Brassica napus anther culture. Z.Pflanzenzuchtg. 80, 100-108.

Keller, W. A. and K. C. Armstrong, 1977: Embryogenesis and plantregeneration in Brassica napus anther cultures. Can. J. Bot. 55,1383-1388.

Kott, L. S., Polsoni, L., Beversdorf, W. D. (1988) Cytological aspectsof isolated microspore culture of B. napus. Can. J. Bot. 66, 1658-1664.

Ku, M. K., Chen, W. C., Kuo, L. C., Kuan, Y. L., An, H. P. & Huang, C.H. 1981. Induction factors and morpho-cytological characteristics ofpollen-derived plants in maize (Zea mays).--In Proc. Symposium PlantTissue Culture. pp. 35-41. Pitman Publishing Ltd. and The Sci. Press,Peking, ISBN 0-237-08488-7.

Lau, J. T. Y., Pittenger, M. F., Cleveland, D. W. (1985) Reconstructionof appropriate tubulin and actin gene regulation and-actin genes. Mol.Cellular Biol. 5, 1611-1620.

Levan, A. 1938. Effects of colchicine on root mitosis inAllium.--Hereditas 24:471-486.

Lichter, R. (1981) Anther culture of Brassica napus in a liquid medium.Z. pflanzenphysiol. 103, 229-237.

Lichter, R. 1982. Induction of haploid plants from isolated pollen ofBrassica napus.--Z. Pflanzenphysiol. 105:427-434.

Lignowski, E. M. & Scott, E. G. 1972. Effect of trifluralin onmitosis.--Weed Sci. 20:267-270.

Loh, C.-S. and D. S. Ingram, 1983: The response of haploid secondaryembryoids and secondary embryogeneic tissues of winter oilseed rape totreatment with colchicine. New Phytol. 95, 359-366.

Margolis, R. L., Wilson, L. (1977) Addition of colchicine-tubulincomplex to microtubule ends: the mechanism of substoichiometriccolchicine poisoning. Proc. Natl. Acad. Sci. 74, 3466-3470.

Mathias, R., and G. Robbelen, 1991: Effective diploidization ofmicrospore-derived haploids of rape (Brassica napus) by in vitrocolchicine treatment. Plant Breeding 106, 82-84.

Mineyuki, Y., Gunning, B. E. S. (1990) A role for preprophase bands ofmicrotubules in maturation of new cell walls, and a general proposal onthe function of 4 preprophase band sites in cell division in higherplants. J. Cell Sci. 97, 527-537.

Morejohn, L. C. & Fosket, D. E. 1984. Taxol-induced rose microtubulepolymerization in vitro and its inhibition by colchicine.--J. Cell Biol.99:141-147.

Morejohn, L. C., Bureau, T. E., Tocchi, L. P. & Fosket, D. E. 1984.Tubulins from different higher plant species are immunologicallynonidentical and bind colchicine differentially.--Proc. Natl. Acad. Sci.81:1440-1444.

Newell, C. A., Rhoads, M. L. & Bidney, D. L. 1984. Cytogenetic analysisof plants regenerated from tissue explants and mesophyll protoplasts ofwinter rape, B. napus L.--Can. J. Genet. Cytol. 26:752-761.

Pechan, P. M., Bartels, D., Brown, D. C. W., Schell, J. (1991)Messenger-RNA and protein changes associated with induction of Brassicanapus embryogenesis. Planta 184, 17, 161-165.

Pechan, P. M., Keller. W. A. (1989) Induction of microsporeembryogenesis in Brassica napus L. by gamma irradiation and ethanolstress. In Vitro Cell Dev. Biol. 25, 1073-1075.

Probst, G. W., Golab, T. & Wright, L. 1976. Dinitroanilines.--InHerbicides. (P. C. Kearney and D. D. Kaufman, eds), pp. 453-500. MarcellDekker Inc., New York.

Quader, H. & Filner, P. 1980. The action of antimytotic herbicides onflagellar regeneration in Chlamydomonas reinhardii: a comparison withthe action of colchicine.--European. J. Cell Biol. 21:301-304.

Simmonds, D. H. (1986) Prophase bands of microtubules occur inprotoplast cultures of Vicia hajastana Grossh. Planta 167, 469-472.

Simmonds, D. H. (1994) Mechanism of induction of microsporeembryogenesis in Brassica napus: significance of the preprophase band ofmicrotubules in the first sporophytic division. In: Biomechanics ofactive movement and division of cells. NATO ASI series, Vol. H 84,Akkas, N., ed. pp. 569-574, Springer-Verlag, Berlin.

Simmonds, D. H., Gervais, C., Keller, W. A. (1991) Embryogenesis frommicrospores of embryogenic and non-embryogenic lines of Brassica napus.In: Proceedings GCIRC 8th International Rapeseed Congress, pp. 306-311,McGregor D. I., ed. Saskatoon, Sask, Canada.

Simmonds, D. H. & Setterfield, G. 1986. Aberrant microtubuleorganization can result in genetic abnormalities in protoplast culturesof Vicia hajastana Grossh.--Planta 167:460-468.

Simmonds, D. H., Seagull, R. W. & Setterfield, G. 1985. Evaluation oftechniques for immunofluorescent staining of microtubules in culturedplant cells.--J. Hist. and Cytochem. 33:345-352.

Srivastava, P. S., Johri, B. M. (1988) Pollen embryogenesis. J.Palynology. 23, 83-99.

Sternlicht, H., Ringel, I., Szasz, J. (1983) Theory for modelling thecopolymerization of tubulin and tubulin-colchicine complex. Biophys. J.42, 255-267.

Subrahmanyam, N. C. and K. J. Kasha, 1975: Chromosome doubling of barleyhaploids by nitrous oxide and colchicine treatment. Can. J. Genet.Cytol. 17, 573-583.

Sunderland, N., Collins, G. B. & Dunwell, J. M. 1974. The role ofnuclear fusion in pollen embryogenesis of Datura innoxia Mill.--Planta117:227-241.

Swanson, E. B., Herrgesell, M. J., Arnoldo, M., Sippell, D. W. Wong, R.S. C. (1989) Microspore mutagenesis and selection: Canola plants withfield tolerance to the imidazolinones. Theor. Appl. Genet. 78, 525-530.

Swanson, E. R. 1990. Microspore culture in Brassica.--In Methods inMolecular Biol. Vol. 6: Plant Cell and Tissue Culture Techniques. (J. W.Pollard and J. M. Walker eds), pp. 159 169. Humana Press, ISBN0-685-38212-5.

Telmer, C. A., Newcomb, W., Simmonds, D. H. (1994) Cellular changesduring heat shock induction and embryo development of culturedmicrospores of Brassica napus cv. Topas. Protoplasma. In Press.

Telmer, C. A., Newcomb, W., Simmonds, D. H. (1993) Microsporedevelopment in Brassica napus and the effect of high temperature ondivision in vivo and in vitro. Protoplasma. 172, 154-165.

Telmer, C. A., Simmonds, D. H., Newcomb, W. (1992) Determination ofdevelopmental stage to obtain high frequencies of embryogenicmicrospores in Brassica napus. Physiologia Plantarum 84, 417-424.

Terasaka, O., Niitsu, T. (1990) Unequal cell division and chromatindifferentiation in pollen grain cells II. Microtubule dynamicsassociated with the unequal cell division. Bot. Mag. Tokyo 103, 133-142.

Van Lammeren, A. A. M., Keijzer, C. J., Willemse, M. T. M., Kieft, H.(1985) Structure and function of the microtubular cytoskeleton duringpollen development in Gasteria verrucosa (Mill.) H. Duval. Planta 165,1-11.

Vierling, E. (1991) The roles of heat shock proteins in plants. Annu.Rev. Plant Physiol Plant Mol. Biol. 42, 579-620.

Wan, Y., J. F. Petolino, and J. M. Widholm, 1989: Efficient productionof doubled haploid plants through colchicine treatment of anther-derivedmaize callus. Theor. Appl. Genet. 77, 889-892.

Wenzel, G., F. Hoffmann, and E. Thomas, 1976: Heterozygousmicrospore-derived plants in rye. Theor. Appl. Genet. 48, 205-208.

Wenzel, G., Hoffmann, F. & Thomas, E. 1977. Anther culture as a breedingtool in rape: I. ploidy level and phenotype of androgenic plants.--Z.Pflanzenzuchtg. 78:149-155.

Wong, C. K. 1989. A new approach to chromosome doubling for haploid riceplants.--Theor. Appl. Genet. 77: 149-151.

Zaki, M. A. M., Dickinson, H. G. (1991) Microspore-derived embryos inBrassica: the significance of division symmetry in pollen mitosis I toembryogenic development. Sex. Plant Reprod. 4, 48-555.

Zhao, J.-P. and D. Davidson, 1984: Distribution of chromosomes intodiscrete group in colchicine-induced C-metaphases of barley. Caryologia37, 331-342.

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A method of inducing embryogenesis from plant microspores comprising:treating a microspore plant culture with colchicine at about 25° C.; and incubating for a sufficient time to induce embryogenesis.
 2. The method of claim 1 wherein the method further induces chromosome doubling.
 3. The method of claim 2 wherein the colchicine is from about 12.5 μM to about 100 μM.
 4. The method of claim 3 wherein the colchicine is from about 25 μM to about 50 μM.
 5. The method of claim 4 wherein the sufficient time is from about 15 to about 50 hours.
 6. The method of claim 5 wherein the sufficient time is from about 18 to about 42 hours.
 7. The method of claim 6 wherein the method further comprises reducing the concentration of colchicine in the microspore culture and incubating at about 25° C. until an embryo is formed.
 8. The method of claim 7 wherein the microspore plant culture is a Brassica napus culture.
 9. The method of claim 1 wherein the treating step is by culturing in the presence of a medium containing the colchicine.
 10. The method of claim 1 wherein the treating step is by a biolistic delivery system.
 11. A method of inducing embryogenesis and chromosome doubling from plant microspores comprising;treating a microspore plant culture of B. napus with about 25 μM to about 50 μM of colchicine at about 25° C.; incubating the microspore plant culture for about 18 to about 42 hours to induce embryogenesis and chromosome doubling; reducing the colchicine to about 12.5 μM by dilution; and incubating at about 25° C. for 3 to 4 weeks until an embryo is formed. 